Hydrogen is a vital raw material or reducing agent for the fertilizer, substitute natural gas production, oil refining, chemical, and food and materials processing industries. Currently hydrogen is produced by reforming of natural gas, petroleum or coal, by partial oxidation of hydrocarbons or by electrolysis of water. The first two methods consume natural resources and the last method is capital intensive, costly and inefficient. Direct thermal decomposition of water requires impractically high temperatures for significant yields of hydrogen and oxygen and engenders the problem of preventing the oxygen and hydrogen from recombining when the product gases are cooled down for storage and/or distribution.
It has long been known that it is theoretically possible to produce oxygen and hydrogen from water by introducing heat and water into a closed thermochemical cycle involving intermediate chemical compounds. Calculations for such cycles suggest that they can be more efficient in the consumption of process heat than is water electrolysis.
The Euratom Mark I process is one such closed thermochemical cycle which uses intermediate calcium, bromine and mercury compounds and process heat at temperatures exceeding 700.degree. C. As is indicated in U.S. Pat. No. 3,821,358, mercury is highly volatile and its loss to the surrounding atmosphere would pose a significant ecological and health hazard.
Other cycles involve dilute water solutions and serious problems of energy consumption to effect the product separations required. Very high efficiencies during such product separations are required if overall process efficiencies are to be competitive with electrolysis. In such cycles much heat is consumed heating and evaporating solvent water.
It is an object of this invention to provide a closed cycle thermochemical process which presents fewer product separation problems, involves no heating or evaporative separations of dilute solutions of salts, has cycle thermal efficiencies higher than that of electrolysis, and is adaptable to process heat which can be provided by any source of high temperature heat such as nuclear fusion or fission reactors or solar energy collection and concentration systems.